Researchers for the Cassini mission to Saturn today announced three new observations: the detection of lightning in Saturn's atmosphere that is different in character than was previously observed, the discovery of an additional radiation belt above Saturn's cloudtops, and the observation of a fluorescent glow in the upper atmosphere of Titan.
Saturn seen two weeks after Cassini's insertion into orbit around it. Photo credit: Courtesy NASA.
The lighting was observed by the radio and plasma wave science instrument on the space craft as bursts of radio waves, which researchers described as similar to the crackle of static one hears on a radio during a thunderstorm. Lightning was previously observed by the Voyager spacecraft twenty years ago. The lighting characteristics seen by Cassini differ from those seen by Voyager; while Cassini sees great variability from day to day in the rate of lighting strikes, Voyager had seen a regualer rate that persisted for months. Researchers suggested that the difference arises from the orientation of the ring relative to the sun. When Voyager was at Saturn, the ring was edge-on to the sun, so that it cast a strong and narrow shadow at Saturn's equator. This produced a strong temperature variation with latitude, giving rise to long-lived thunderstorm systems. Now the ring is strongly tilted to the sun, so that it casts a very broad and less deep shadow at higher latitudes on Saturn. This should lead to a less-extreme temperature variation with latitude and a less active atmosphere.
The new radation belt was detected by Cassini's magnetospheric imaging instrument. The belt extends from just above Saturn's cloud tops to the edge of Saturn's inner-most ring. The belt was found by detecting fast neutral atoms that were created when ions trapped in the belt interacted with gas between Saturn and its ring.
The sun-induced fluorescent glow of methane and carbon monoxide in Titan's atmosphere was observed with Cassini's visual and infrared mapping spectrometer in both the daytime, which was expected, and the nighttime, which was unexpected. The glow is in the near infrared, and is occurring 700km above the moon's surface.